This invention relates to a differential amplifier and more particularly to a differential amplifier suitable for use in an MIS integrated circuit constituted of MISFETs (insulated-gate fieldeffect transistors).
A differential amplifier includes a differential input stage which consists of symmetric circuits, and can eliminate the in-phase component of differential inputs. Noise that is liable to be superimposed on a power source voltage can be regarded as in-phase noise by the differential amplifier, hence a differential amplifier has the advantage that the power source noise does not influence the output of the differential amplifier. In an integrated circuit including a logic circuit together with a substantially analog circuit, the power source voltage is likely to vary according to the logic operation of the circuit. Since a differential amplifier is virtually insensitive to power source noise, it can be fabricated extremely conveniently in an integrated circuit configuration together with the logic circuit.
With a differential amplifier, however, the output fluctuates even if the differential inputs are at the same level, when there is no stabilization point determined by the circuit itself. In other words, the differential amplifier has the problem that its operating point is not stabilized.